[Rd] Fix documentation about <- and = differences

[Duncan Murdoch]

On 2025-01-04 1:25 a.m., Philip Rogers wrote:
> Hello R devs,
> 
> this StackOverflow answer says that the R documentation is wrong:
> https://stackoverflow.com/a/51564252/1719931
> 
> Can we fi it?

By the way, the SO answer is wrong about the history of this.  It says 
that this message:  https://developer.r-project.org/equalAssign.html is 
the "original explanation by John Chambers", but in fact John committed 
the text that is still present in the docs several months earlier,
to both the `?assign` help page (rev 15467, 2001-08-13) and to the 
equalAssign.html document on developer.r-project.org (rev 608, same day).

Duncan Murdoch